1. Field of the Invention
This invention relates to a mobile radio communication control system, in which many mobile stations perform tansmission and reception of a control signal between each of them and a fixed control station over a common two-way control channel, and, more particularly, to an idle channel control system in which, to prevent simultaneous transmission from a plurality of mobile stations, information indicative of busy or idle state of the control channel from the mobile stations to the fixed control station, that is, a reverse direction channel, is transmitted to the mobile stations through the control channel from the fixed control stations to the mobile stations, that is, a forward direction channel.
2. Description of the Prior Art
In mobile radio communication, it is customary to employ two radio frequencies as a common two-way control channel between each of many mobile stations and the fixed station. In a case where a plurality of mobile stations simultaneously start respective calls, and each performs the operation of transmitting a control signal by emitting the radio frequency wave for the reverse direction channel of the common two-way control channel, interference occurs in the common reverse direction channel, making it impossible for the fixed station to correctly receive the control signal from any of the mobile stations. To prevent this, an idle channel indication signal indicating the idle state of the reverse direction channel is always transmitted from the fixed station by the radio frequency wave of the forward direction channel of the common two-way control channel, and each mobile station transmits the control signal to start communication only when receiving the above-said idle channel indication signal.
In this case, the mobile station emits at first an electric wave for the reverse direction channel, and after a certain period of time, transmits a start signal indicating the start of the control signal and then the control signal. After the control signal has been transmitted, the electric wave for the control signal transmission is stopped, and necessary control operations are performed from instruction from the fixed station, so that communication is enabled between the mobile station and the fixed station through an electric wave for the voice signal transmission. And, upon initiation of emitting an electric wave from the mobile station, the electric wave is detected by a radio receiver of the fixed station. This detection requires a certain amount of time; namely, a detecting pulse is obtained after T.sub.1 seconds from the emission of electric wave from the mobile station. This T.sub.1 seconds will hereinafter be referred to as the electric wave detection time. In the electric wave detection time T.sub.1 at the fixed station, the idle channel indication signal is transmitted over the forward direction channel in spite of the electric wave transmission from the mobile station, and if some other station emits an electric wave for transmitting the control signal over the reverse direction channel, interference occurs as mentioned previously. Accordingly, in order to lessen the probability of occurrence of such interference to perform the idle channel control as completely as possible, it is necessary to minimize the electric wave detecting time T.sub.1.
In the above-mentioned idle channel control system, if one radio service zone is controlled by one fixed station, the transmission of the idle channel signal can be interrupted at the same time as the radio wave is detected by the fixed station. Accordingly, a time delay from the start of the transmission of the radio wave from the mobile station to the interruption of the idle channel indication signal is substantially equal to the time T.sub.1.
In mobile radio communication, if the whole service area expands into a large radio service zone, since the transmitting power of each mobile station is limited, it is necessary to divide the large radio service zone into a plurality of small radio service zones and constitute the fixed station side with a plurality of base stations respectively corresponding to the small radio service zones and one control station for commonly controlling the base stations. Each base station and the control station are respectively interconnected through a plurality of fixed transmission lines. In this case, the small radio service zones are each established to have such an area that an electric wave from each of the mobile stations of each small radio service zone can be received by the corresponding base station with a sufficient electric field intensity. On the other hand, an electric wave from the side of the fixed station is transmitted with a high power from the control station so that an electric wave may be received by any of the mobile stations of the large radio service zone with a sufficient electric field intensity.
In the large radio service zone constructed as described above, if the above-said idle channel control is practiced, when each base station detects the electric wave from the mobile station of its small radio service zone, it is necessary to transmit a detection pulse indicative of the electric wave detection to the control station for controlling therein the transmission of the idle channel indication signal. Let a time necessary for transmitting the above-said detection pulse be taken as T.sub.2, then there is the possibility of the aforementioned simultaneous transmission from a plurality of mobile stations for a period of time equal to the sum (T.sub.1 + T.sub.2) of the electric wave detection time T.sub.1 in the base station and the above time T.sub.2. Accordingly, the time T.sub.2 is also required to be as short as possible.
There are the following methods for transmitting the electric wave detection pulse:
(1) Since a fixed transmission line is usually laid between each base station and the control station for the control signal transmission from the mobile station, this transmission line is used for transmitting both of the control signal and the electric wave detection pulse.
(2) A transmission line for the exclusive use of the electric wave detection pulse is laid separately of the control signal transmission line.
With the method (1), to distinguish between the control signal and the electric wave detection pulse, the signal duration of the former is required to be substantially equal to the latter so that the detection pulse transmission time T.sub.2 becomes too long, resulting in the loss of effective meaning of the idle channel control. Further, the method (2) necessitates an extra fixed transmission line, which raises the cost of the overall system. Either of the conventional methods has such a fatal defect.
A wide service area inevitably necessitates a large number of mobile stations, and naturally leads to increase probability of simultaneous transmission. In the prior art, there is no proper means for the transmission of the electric wave detection pulse, so that although the probability of simultaneous transmission from a plurality of mobile stations is high in the large radio service zone, the idle channel control for preventing the simultaneous transmission cannot be carried out.